ports-mgmt/synth: Build with watchdog monitor by default

Without WATCHDOG Synth will hang on the ports that stop building [1].

While here:

• Rename the double negative NOWATCHDOG option to WATCHDOG

[1] https://github.com/jrmarino/synth/issues/53#issuecomment-335425660

PR: 222900
Submitted by: ericturgeon.bsd@gmail.com (maintainer)
MFH: 2017Q4